The present invention relates to a charge coupled device image sensor and a method of making the same.
A charge coupled device (CCD) image sensor essentially comprises a light receiving region for receiving light and generating a signal charge in response to the received light, and a signal charge transfer region for transferring the generated signal charge in a single direction.
However, the light receiving region generally generates excess signals charge when too much light is incident on the light receiving region.
The excess signal charge causes a blooming phenomenon to be generated on a screen.
When a signal charge in excess of necessary signal charges is moved from the light receiving portion to the signal charge transfer region, this phenomenon is referred to as an over flow drain (OFD).
Conventionally, the light receiving region of CCD image sensor has a structure for preventing the OFD phenomenon.
The structure of the light receiving region for preventing the OFD phenomenon may generally be classified into a lateral structure and a vertical structure.
At present it is preferable to use the vertical structure, as the size of the light receiving region is reduced in accordance with the high integration of CCD image sensor.
Referring to FIG. 1, there is shown a section of a conventional CCD image sensor which has a light receiving region of the vertical structure to prevent the OFD phenomenon.
As shown in FIG. 1, the conventional CCD image sensor comprises: a silicon substrate 1 of n-type;
a well 2 of p-type formed in the surface of the silicon substrate 1; a first insulation film 3 formed on the surface of the well 2; PA1 a signal charge transfer region 4 of n-type formed in the surface of the well 2; a surface layer 5 of p.sup.+ -type (.sup.+ denotes a high concentration) spaced from the signal charge transfer region 4 with a constant distance, for applying an initial bias; PA1 a light receiving region 6 of n-type formed beneath the surface layer 5 of p.sup.+ -type; a first electrode 7 formed on the surface of the first insulation film 3 which is corresponding to the upper part of the signal charge transfer region 4; a second insulation film 8 formed on the surface of the first electrode 7; PA1 a second electrode 9 formed on the surface of the second insulation film 8 and the surface of the first insulation film 3 which is located between the signal charge transfer region 4 of n-type and the light receiving region 6 of n-type; PA1 a third insulation film 10 formed on the surface of the second electrode 9; and PA1 a light shield layer 11 formed on the surface of the first insulation film 3 and the third insulation film 10 except for the upper portion of the light receiving region 6 of n-type. PA1 a semiconductor substrate of a first conductivity type connected to a ground; PA1 an impurity region of a second conductivity type formed in the surface of the semiconductor substrate of the first conductivity type, to serve as a blooming prevention layer; PA1 an impurity region of the first conductivity type formed in the surface of the semiconductor substrate, so that it encloses the impurity region of the second conductivity type serving as a blooming prevention layer, to serve as a potential barrier layer; PA1 an impurity region of the second conductivity type formed in the surface of the semiconductor substrate of the first conductivity type, so that it encloses the impurity region of the first conductivity type serving as a potential barrier layer, to serve as a light receiving region; PA1 an insulation film which is formed on the surface of the semiconductor substrate of the first conductivity type and has contact holes at both edges of the impurity region of the second conductivity type, serving as a blooming prevention layer; PA1 silicide films filled in the contact holes; and PA1 a light shield conductor film which is formed on the surface of the remaining insulation film, except for a portion between the silicide films and the surfaces of the silicide films, and is connected to a voltage source. PA1 forming a first insulation film on the whole surface of a semiconductor substrate of a first conductivity type; PA1 implanting impurity ions of the first conductivity type having a concentration higher than that of the semiconductor substrate in the semiconductor substrate of the first conductivity type, thereby forming a first channel stop region and a second channel stop region of the first conductivity type spaced from each other with a constant distance in the surface of the semiconductor substrate; PA1 implanting impurity ions of a second conductivity type in the semiconductor substrate of the first conductivity type, thereby forming a signal charge transfer region of the second conductivity type contacted with the first channel stop region, in the semiconductor substrate of the first conductivity type; PA1 forming a conductor on the whole surface of the first insulation film and patterning the conductor, thereby forming a first electrode in the upper portion of the signal charge transfer region; PA1 forming a second insulation film on the whole surfaces of the first electrode and the first insulation film, and patterning the second insulation film, thereby allowing only a portion of the second insulation film which is formed on the surface of the first electrode to remain; PA1 forming a conductor on the whole surface of the remaining second insulation film and the first insulation film and patterning the conductor, thereby allowing only a portion of the conductor which is formed on the surface of the remaining second insulation film to remain, as a second electrode; PA1 implanting impurity ions of the second conductivity type in the semiconductor substrate of the first conductivity type, thereby forming a light receiving region of the second conductivity type in the surface of the semiconductor substrate between the second channel stop region and the lower portion of the second electrode; PA1 implanting impurity ions of the first conductivity type in the light receiving region of the second conductivity type, thereby forming a potential barrier layer of the first conductivity type in the light receiving region of the second conductivity type; PA1 implanting impurity ions of the second conductivity type having a concentration higher than that of the light receiving region in the potential barrier layer of the first conductivity type, thereby forming a blooming prevention layer of the second conductivity type; forming a third insulation film on the whole surfaces of the first insulation film and the second electrode and patterning the third insulation film, thereby allowing to remain only a portion of the third insulation layer which is formed on the surface of the second electrode; PA1 patterning the first insulation film, thereby forming contact holes respectively at both edge portions of the blooming prevention layer of the second conductivity type; filling the contact holes with a metal having a high melting point; PA1 annealing the metal filled in the contact holes, thereby converting the metal into a silicide film; PA1 forming a conductor on the whole surfaces of the first insulation film, the third insulation film and the silicide films and patterning the conductor so that only a portion between the silicide films can be removed, thereby forming a light shield conductor layer; and PA1 forming a protection film on the exposed whole surfaces of the first insulation film and the light shield conductor layer.
Herein, the well 2 of p-type comprises a shallow well 2a and a deep well 2b to prevent a smear phenomenon.
The smear phenomenon means that a part of the signal charges generated from a light receiving region corresponding to a pixel are mixed with signal charges generated from another light receiving region corresponding to another pixel, and then moved to the light receiving region of n-type, corresponding to the other pixel.
Consequently, the smear phenomenon makes it impossible to correctly display an image on a screen and moreover reduces the resolution of the screen.
The operation of the CCD image sensor shown in FIG. 1 will hereinafter be described briefly in conjunction with FIG. 1.
First, when light corresponding to an image signal is incident, the light receiving region 6 generates signal charges corresponding to the image signal.
At this time, if too much light is incident on the light receiving region 6, the light receiving region 6 of n-type generates unnecessary excess signal charges in excess of the signal charges corresponding to the image signal.
At this time, if a bias is applied to the silicon substrate 1 of n-type which is connected to a voltage source Vd, the saddle point of potential taken on the line A-A' is then lowered toward the silicon substrate 1 of n-type by a predetermined height.
Therefore, the excess signal charges flow toward the silicon substrate 1, thereby enabling the OFD phenomenon and the smear phenomenon to be reduced.
However, the above-conventional technique has the following disadvantages.
First, the time for making a CCD image sensor is increased and the method of making a CCD image sensor is complicated, since a p-type well is essentially formed to prevent the OFD phenomenon.
Second, it is easy for the smear phenomenon to be generated and quantum efficiency is low, since the characteristic of long wavelength is not good.